Chapter 8. Crates and Modules

Suppose you’re writing a program that simulates the growth of ferns, from the level of individual cells on up. Your program, like a fern, will start out very simple, with all the code, perhaps, in a single file—just the spore of an idea. As it grows, it will start to have internal structure. Different pieces will have different purposes. It will branch out into multiple files. It may cover a whole directory tree. In time it may become a significant part of a whole software ecosystem.

This chapter covers the features of Rust that help keep your program organized: crates and modules. We’ll also cover a wide range of topics that come up naturally as your project grows, including how to document and test Rust code, how to silence unwanted compiler warnings, how to use Cargo to manage project dependencies and versioning, how to publish open source libraries on crates.io, and more.

Crates

Rust programs are made of crates. Each crate is a Rust project: all the source code for a single library or executable, plus any associated tests, examples, tools, configuration, and other junk. For your fern simulator, you might use third-party libraries for 3D graphics, bioinformatics, parallel computation, and so on. These libraries are distributed as crates (see Figure 8-1).

Figure 8-1.
A crate and its dependencies

The ...

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